[GitHub] [tvm-rfcs] sjoerdmeijer commented on pull request #18: [RFC] Adding initial SVE implementation

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sjoerdmeijer commented on pull request #18:
URL: https://github.com/apache/tvm-rfcs/pull/18#issuecomment-910235468


   @masahi, about: 
    
   > If possible, I think it is better not to introduce user facing changes, since as far as an API is concerned, `s[C].vectorize(...)` is already vector-length agnostic.
   
   which I think is very closely related to an earlier inline comment:
   
   > As I commented above, I'd like to continue using s[C].vectorize(...) and when the feature is available, enable SVE by a target attribute. So I don't expect any user facing work.
   
   I think we do need a user-facing option to toggle fixed/scalable vectorisation. If the vectorisation strategy is selected based on an available target attribute, we loose control to choose fixed/scalable vectorisation. For architectures that do support scalable vectorisation, fixed width might still be preferable in some cases.
   
   I think this is similar to Clang's loop pragmas. For example, the `vectorise_width` pragma has been extended with an optional second argument `fixed|scalable`:
   
   `vectorize_width(_value_[, fixed|scalable]),`
   
   see also the Clang docs [here](https://clang.llvm.org/docs/LanguageExtensions.html#extensions-for-loop-hint-optimizations).
   
   So I see two approaches:
   - we extend `s[C].vectorize(...)` to take an optional fixed/scalable boolean value, similar to Clang's loop pragma, which defaults to fixed if omitted, 
   - or introduce `s[C].vectorize_scalable(...)` as proposed in this RFC.
   
   I personally don't have any preference. But now I am wondering if extending `s[C].vectorize(...)`, the first option, would be better. What do you think?


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